Data storage systems typically arrange the data and metadata of file systems in blocks of storage. For example, the file data constituting files in a file system are stored in blocks of storage, as are inodes, indirect blocks, and other metadata. Data storage systems may provision storage to file systems in units of fixed size, here called “slices.” Data storage systems may generate slices, for example, from one or more physical storage devices, such as RAID groups of physical storage devices.
Some data storage systems provide thinly provisioned file systems. Thinly provisioned file systems typically have very large address spaces but allocate specific storage slices to populate file systems only as storage is needed to satisfy write requests. A thinly provisioned file system may thus have an address space that is measured in petabytes but may allocate slices to occupy only a small fraction of the address space.
Data storage systems that provide thinly provisioned file systems may deallocate blocks of storage from the file systems when the blocks are no longer used, as part of file system shrink operations. In one kind of shrink operation, a data storage system identifies free blocks of storage in the slices supporting the file system. Any completely freed slices may be returned to a storage pool for later reuse.